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close this bookAsbestos Overview and Handling Recommendations (GTZ, 1996)
close this folderPart III. Asbestos substitutes
close this folder4 Fiber-free substitutes in construction area
View the document4.1 Fiber-free substitutes in housing construction
View the document4.2 Fiber-free substitutes in water mains construction

4.2 Fiber-free substitutes in water mains construction

The properties of materials used in water mains construction were already introduced in Section 4.1. Consequently, they are mentioned below only with respect to their application as pipe material.

4.2.1 Metal pipes

· Steel Pipes

Steel pipes have a high strength with great fracture strain and a high notch impact strength. Additionally, a shifting of forces and tensions is possible. The installation of long pipes and the line installation are feasible. Adjustment to the local conditions and heights by cutting and welding in situ as well as simple manufacturing of pipeline fittings and axial force closing joints make this material very popular.

Application occurs primarily where high strength and fracture strains are required, high inner pressures and high pressure surges are likely, and where large pipe sizes occur, for instance at supplier and long-distance pipelines and at special constructions, such as inverted siphons or intersections. Initially, seamless steel pipes were used, and with improvement in welding techniques, welded pipes have been applied increasingly. The seamless and welded pipes are available up to DN 500, above DN 600 only welded pipes.

Disadvantageous are the required corrosion protection measures, such as encasement and lining, since there is a high corrosion probability with insufficient encasement and inadequate protective lining. During transport, installation and bedding, additional measures for protection of the pipe wrapping are needed. For corrosion protection in the interior, cement mortar lining ( DIN 2614) has proven effective. The earlier and still frequently typical bituminous coating has not proven successful. In pipeline fittings the cement mortar lining still presents problems, however. For the outer protection the polyethylene wrapping is used in general. Longer steel pipes are frequently protected cathodically. The electrical conductivity can be either advantageous or disadvantageous, depending on the application area.

· Cast iron pipes

The gray cast iron pipe has been in use for the past 500 years and was previously cast horizontally in two-part sand moulds and later standing in sand moulds. Since 1926 centrifugal casting has become prevalent. Pipeline fittings continue to be cast in sand forms. The important material progress occurred in 1965, when the ductile cast was introduced. The gray cast has a brittle fracture behavior, due to its morphology. However, through special melt additives one acquires the ductile cast, which is superior by its elevated fracture strain coupled with high strength.

The cast iron pipes are offered in 3 classes for water pipes, depending on the nominal widths DN. The corresponding permissible nominal pressure is determined from the nominal width.

The simple manufacturing of the pipe joints is advantageous, as are the bending ability and axial mobility in the joints. Particular measures are required for the uptake of the axial strengths, however. The pipe joints are usually not electrically conductive. Pipeline fittings are available in a wide sortiment.

With the change to ductile cast iron, the corrosion resistance of the gray cast iron has been lost, and hence the pieces must be protected like steel. The cement mortar coating (DIN 2614) is typically used for the inner corrosion protection. Suitable for the outer protection are: PE-encasement (manufacturing plant), cement mortar encasement (manufacturing plant), and PE-foil encasements for joints at the construction site; galvanizing by spraying with a bituminous covering or only bituminous covering may only be applied by proven non-aggressive soils.

4.2.2 Plastic pipes

In water supply mains, primarily polyvinyl chloride (PVC, free of softeners) and polyethylene (LDPE, HDPE) are applied. PVC is used mainly in distribution nets, while HDPE/LDPE-pipes are specially used for house connections and for supply lines of smaller nominal widths, e.g. in rural areas. The properties of both pipe types can be characterized as follows:

· PVC, softener-free

An advantage is the resistance to corrosion, the low weight, the simple manufacturing of the elastically sealed plug socket joints, the axial mobility of the plug socket joints and a very smooth pipe wall.

Difficulties can arise with temperature fluctuations and extreme climate conditions. The sustainable tensions can become reduced depending on temperature and operation time. The impact sensitivity increases at temperatures c 5°C. Particular care must be taken during the bedding of the pipes. PVC pipes are very sensitive to external stress. Generally, there is a very low bending ability in the pipe joints. Furthermore, transition problems sometimes occur, since often the only pipeline fittings available are made of other materials.

· LDPE (Low density PE), HDPE (High density PE)

LDPE/HDPE-pipes are also very corrosion resistant and are often used because of their low weight. They are bendable (particularly LDPE) and are available as endless pipes, which are available on a ring or drum. Relatively long pipes are feasible which do not need bends and can be installed in narrow pipe beds. A very smooth pipe wall and an axial strength joint support the application of these pipes.

As with PVC pipes, the sustainable tensions are reduced in correlation with the temperature and operation time. At low temperatures marked stiffening can arise. The pipes are also very sensitive against local peak tensions (e.g. caused by sharp-edged stones or damage to the surface). Furthermore, the high thermal coefficient of expansion is also disadvantageous. In some cases, only pipe joints made of other materials are available.

4.2.3 Concrete pressure pipes

Concrete pressure pipes are mostly applied for supplying and long-distance pipelines having large nominal widths with low to medium pressures and few built-in components and fittings. The long period of use and the high resistance against inner and outer corrosion are particular advantages of this type of pipe. Furthermore, the static measurement can be determined exactly according to the local loads. The high resistance to denting is also positive.

Negative aspects are the high weight and the brittleness of the pipe material. A good deal of technical knowledge and knowledge of statics are needed, because no axial strength joints are possible, the fittings generally not being pre-tensed, but designed constructively in another fashion or available in steel. Difficulties can also arise through later manufacturing of bonds and through repairs. During the installation great care must be taken, particularly in making the bottom of the ditch and the pipe joints.

Nowadays the following concrete pipe types are primarily used:

· Reinforced Concrete Pressure Pipes

Reinforced concrete pressure pipes are mostly used as pressure pipes, protecting pipes or jacket pipes for holing-through and underground driving as well as sewer pipelines with pressures up to PN 2.5 bar. They are manufactured as jolted, rolled and spun concrete pipes with DN 250-4000 (and larger), lengths of 2.5-Sm and with unstressed ring and axial reinforcement.

· Prestressed Concrete Pipes

The reinforcement is prestressed in the ring direction, for which the winding process and prestressing through core expansion are available. Generally, prestressing of the axial reinforcement is also performed. The diameters lie between DN 400 and 2500, the wall thicknesses are 55-150 mm, the construction length is 5m. The concrete quality of B 55 should be met or exceeded; typically a concrete strength 100 N/mm² is attained.